The present disclosure relates generally to switched reluctance machines, and more particularly, to sensorless rotor position detection for switched reluctance machines.
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized where the inductance of the exciting winding is maximized. In one type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This type of reluctance machine is generally known as a switched reluctance machine. It may be operated as a motor or a generator. The characteristics of such switched reluctance machines are well-known and are described in, for example, “The Characteristics, Design and Application of Switched Reluctance Motors and Drives” by Stephenson and Blake, PCIM '93, Nurnberg, Jun. 21-24, 1993, incorporated herein by reference. That paper describes the features of the switched reluctance machine produce the characteristic cyclically varying inductance of the phase windings.
The principal components of a typical switched reluctance drive system include a DC power supply, for example, a battery or rectified and filtered AC supply that can be fixed or variable in magnitude. The DC voltage provided by the power supply is switched across the phase windings of the motor by a power converter under the control of an electronic control unit. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector is typically employed to supply signals indicating the angular position of the rotor. The output of the rotor position detector may also be used to generate a speed feedback signal. Current feedback is provided in the controller by a current transducer that samples current in one or more of the phase windings.
The rotor position detector may take many forms. In some systems, the rotor position detector can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter is required. In other systems, the position detector can be a software algorithm that calculates or estimates the position from other monitored parameters of the drive system. These systems are often called “sensorless position detector systems” since they do not use a physical transducer associated with the rotor that determines the angular position of the rotor. Many different approaches have been adopted in the quest for a reliable sensorless system.
The energization of the phase windings in a switched reluctance machine depends on detection of the angular position of the rotor. This may be explained by reference to FIGS. 1 and 2, which illustrate the switching of a reluctance machine operating as a motor. FIG. 1 generally shows a rotor 24 with a rotor pole 20 approaching a stator pole 21 of a stator 25 according to arrow 22. As illustrated in FIG. 1, a portion 23 of a complete phase winding is wound around the stator pole 21. When the portion 23 of the phase winding around stator pole 21 is energized, a force will be exerted on the rotor, tending to pull rotor pole 20 into alignment with stator pole 21. FIG. 2 generally shows typical switching circuitry in the power converter that controls the energization of the phase winding, including the portion 23 around stator pole 21. When switches 31 and 32 are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of lamination geometry, winding topology and switching circuitry are known in the art: some of these are discussed in the incorporated Stephenson and Blake paper cited above. When the phase winding of a switched reluctance machine is energized in the manner described above, the magnetic field set up by the flux in the magnetic circuit gives rise to the circumferential forces which, as described, act to pull the rotor poles into line with the stator poles.
In general, the phase winding is energized to effect rotation of the rotor as follows. At a first angular position of the rotor (called the “turn-on angle”, TON), the controller provides switching signals to turn on both switching devices 31 and 32. When the switching devices 31 and 32 are on, the phase winding is coupled to the DC bus, causing an increasing magnetic flux to be established in the machine. The magnetic flux produces a magnetic field in the air gap that acts on the rotor poles to produce the motoring torque. The magnetic flux in the machine is supported by the magneto-motive force (“mmf”), which is provided by a current flowing from the DC supply through the switches 31 and 32 and the phase winding. Current feedback is generally employed and the magnitude of the phase current is controlled by chopping the current by rapidly switching one or both of switching devices 31 and/or 32 on and off. FIG. 3a shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. In motoring operation, the turn-on angle TON is often chosen to be the rotor position where the center-line of an inter-polar space on the rotor is aligned with the center-line of a stator pole, but may be some other angle.
In many systems, the phase winding remains connected to the DC bus (or connected intermittently if chopping is employed) until the rotor rotates such that it reaches what is referred to as the “freewheeling angle” TFW. When the rotor reaches an angular position corresponding to the freewheeling angle (the position shown in FIG. 1), one of the switches, for example 31, is turned off. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example 32) and through only one of the diodes 33/34 (in this example 34). During the freewheeling period, the voltage drop across the phase winding is small, and the flux remains substantially constant. The circuit remains in this freewheeling condition until the rotor rotates to an angular position known as the “turn-off angle” TOFF, (when the center-line of the rotor pole is aligned with that of the stator pole). When the rotor reaches the turn-off angle, both switches 31 and 32 are turned off and the current in phase winding 23 begins to flow through diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease. It is known in the art to use other switching angles and other current control regimes.
As the speed of the machine rises, there is less time for the current to rise to the chopping level, and the drive is normally run in a “single-pulse” mode of operation. In this mode, the turn-on, freewheel and turn-off angles are chosen as a function of, for example, speed and load torque. Some systems do not use an angular period of freewheeling switches 31 and 32 are switched on and off simultaneously. FIG. 3b shows a typical such single-pulse current waveform where the freewheel angle is zero. It is well-known that the values of turn-on, freewheel and turn-off angles can be predetermined and stored in some suitable format for retrieval by the control system as required, or can be calculated or deduced in real time.
In low speed operation, most known sensorless position detection systems that are suitable for operation in the chopping mode use diagnostic pulses of some sort that are injected into an idle, or “inactive” phase winding (no phase excitation current applied to the winding). By monitoring the result of these pulses, the control system is able to estimate the rotor position and determine when the main excitation should be applied to and removed from the phase windings.
As the speed rises, the time remaining for diagnosis becomes inadequate to inject sufficient pulses for a reliable estimate of position, and the system becomes unstable because there are times when none of the phases is in a condition for diagnosis and synchronism of the control system with rotor position is lost. Rather than use diagnostic pulses, high-speed detection systems may take readings from the phase energization waveform at a predetermined reference point, then appropriate corrections are made. One such method for operating in the high-speed (single-pulse) mode is described in EP-A-0573198 (“Ray”), which is incorporated herein by reference. Ray discloses a method of flux and current measurement leading to predictions of rotor position.
Many other sensorless position detection systems are reviewed and categorized in “Sensorless Methods for Determining the Rotor Position of Switched Reluctance Motors”, Ray et al, Proc EPE'93 Conference, Brighton, UK, September 13-16, 93, Vol 6, pp 7-13, incorporated herein by reference, which concludes that none of these methods is entirely satisfactory for operation over the entire operating range.
The present application addresses these and other shortcomings associated with the prior art.